Wells in compactable sediments (or tectonically active areas) are subject to deformation over the productive life of the field. The result is the catastrophic loss of producing zones up to and including the loss of a whole well. The problem is exacerbated by the increasingly rapid off-take rates and the completion of multiple zones in a single well. The observable phenomenon is that at first a well casing will bend or begin to buckle, frequently at casing joints, or interfaces in the formation. As the compaction continues, the movement results in a significant misalignment of the well axis. The result can be the complete loss of the well investment resulting in, not only deferred and/or lost production, but even the replacement cost of a well, which is extremely expensive, particularly in deep water. The ability to detect early bending would warn of later buckle or collapse and allow for changes in production practices and/or remedial action. Monitoring compaction in-situ becomes a complex problem. It can be a billion-dollar issue in some highly compressible formations, as wells and sometimes entire fields are put at risk.
The art is filled with hybrid combinations of radioactive tagging and casing monitoring technologies (mechanical, EM, acoustic or video televiewers). The big problem is that all the solutions are intermittent and require human intervention to execute the process.
One method of monitoring compaction known in the art is to apply radioactive tags to the casing and/or shooting radioactive bullets into the formation. This method requires logging tools to be run in the well periodically to monitor relative movement. The well is shut in and there is always a risk to the well any time a tool is run in that it might not come back out. See, for example, U.S. Pat. No. 5,753,813 and U.S. Pat. No. 5,705,812.
It is known in the art to use fiber optics to measure temperature, pressure, flow, and acoustics. See, for example, U.S. Pat. Nos. 6,450,037; 6,363,089; 6,354,147; 6,346,702; and 6,252,656. (CiDRA).
It is known in the art to run fiber optic cable for the measurement of strain, however this is a more subtle measurement than compaction. The amount of strain the fiber can withstand before breaking is usually on the order of one percent. The strain that compacting wells undergo is at least several percent. In compaction applications casing may undergo displacements or deformations that locally are much greater than 10%. Furthermore, the strains on the well are not extensional, but are compressional. Trying to compress a fiber is literally like trying to compress a string. It will buckle unless it is very rigidly held. Also, if it is held rigidly and strained the amount that the well would strain under compaction forces, it would crush and fracture the fiber. Such deformations would easily break fibers or elements that are constrained to the casing and caused to strain with the casing. The present inventors have observed that breaking might be avoided with a properly designed, bent shape or loop shaped sensor.
Although several papers have reported on field use of a variety of fiber optic microbend sensors, most have not found wide use in practice (other than for alarm mode or tactile sensing) due to problems associated with erratic response, tolerances of the deformers, mechanical fatiguing of the fiber, and a limited quantitative understanding of the mode problems and radiation loss associated with the use of highly multimode fiber. All such patents relate to microbending optical fiber rather than to macrobending or non-linear buckling. The significant advantage of the latter over the former relates to the predictability and reproducibility, which are difficult at best in microbending, but readily achievable in macro-bending, which employs non-linear buckling. This feature is especially significant in sensors used for making precise measurements over a wide dynamic range.
U.S. Pat. No. 5,321,257 discloses a fiber optic bending, and positioning sensor comprising a fiber optic guide having a light emission surface extending in a thin band on a side of the fiber for at least part of its length, said light emission surface covered by a coating of light absorbent material. The primary applications are in the fields of sports medicine and biometrics. In addition, this patent uses a polymer fiber. Such a sensor would not be suitable for well applications, because: 1) with plastic there is a higher intrinsic loss of light; 2) distances in wells are closer to kilometers than meters; 3) plastic is not suitable for the higher temperatures typical in wells, because at about 150° F. degradation would set in and temperatures near 200° F. would cause failure; 4) the sensor of '257 depends upon attenuation, rather than scattering.
U.S. Pat. No. 5,661,246 discloses an assembly which allows use of fiber optic displacement sensors in a high-temperature environment comprising a rod attached to an underlying surface at one point and guided to move in a selected direction of measurement, wherein the distance between a selected movable location on the rod and a point fixed on the surface is measured using a bent optical fiber having light loss characteristics dependent on that distance.
U.S. Pat. No. 5,818,982 discloses fiber optic sensors wherein the shape of a length of fiber is changed under carefully controlled boundary conditions, providing a reproducible macrobending-induced loss which can be implemented in a variety of highly precise and a wide range of sensor applications.
One can observe that measurement of bend diameter in untreated fiber does not provide the signal to noise and dynamic range in measurement that would make it practical for downhole applications for monitoring compaction. Light lost at each bend adds up quickly and soon results in a signal that is too low to measure properly.
Fiber Bragg gratings are known in the art and are made by laterally exposing the core of a single-mode fiber to a periodic pattern of intense UV light. This creates areas of increased refractive index within the fiber. The fixed index modulation is referred to as a grating. All reflected light signals combine coherently to one large reflection at one wavelength when the grating period is equal to half the input wavelength. Other wavelengths of light are for all intents and purposes transparent. So, light moves through the grating with negligible attenuation or signal variation. Only the Bragg wavelength is affected. Light of the Bragg wavelength is strongly backreflected. Being able to preset and maintain the grating wavelength is what makes fiber Bragg gratings so useful. See “Fiber Bragg Grating” 3M US Online, 27 Nov. 2000.
Conventional fiber Bragg gratings would not be suitable for monitoring compaction in a well casing, because typically in a passive sensing system the reflected wavelength is changed as the gratings are stretched. Fibers or elements that are constrained to the casing and stretched would break under forces of the magnitude of compaction.
In an article titled, “Characteristics of short-period blazed fiber Bragg gratings for use as macro-bending sensors”, APPLIED OPTICS, 41, 631-636 (2002), Baek, S., et al, discuss the characteristics of short-period blazed fiber Bragg gratings for use as macro-bending sensors and that the sensors are able to detect macro-bending with the transmitted power variation of the first side mode in the blazed fiber Bragg grating. This article does not discuss applications.
In a paper titled, “Long-Period Fiber Grating Bending Sensors in Laminated Composite Structures”, SPIE Conference on Sensory Phenomena and Measurement Instrumentation for Smart Structures and Materials, March 1998, San Diego, Calif., SPIE Vol. 3330, 284-292, Du, W., et al, present the experimental result of the effect of bending an over-written long-period fiber grating (LPG) on the transmitted power spectrum and total transmitted power of a light-emitting diode (LED). It was found that the total transmitted power through the LPG decreases linearly with bend curvature within the range from 0 to about 0.001/mm. The error for determining the bend curvature due to the actual non-linearity of the sensor is estimated to be +/−2×10−5/mm. This work appears to use a highly linear attenuation of the light as a function of very slight bend.
In a paper titled, “Ultrastrong Fiber Gratings and Their Applications”, SPIE Conference Phototonics East “Optical Fiber Reliability and Testing”, 3848-26, Sep. 20, 1999, Starodubov, D. S., et al, discuss a method of fabricating gratings for sensors through the polymer coating, wherein the fiber surface is never exposed to the environment, which makes the resulting grating predictably strong.
Nothing has been found in the art that suggests bending a fiber Bragg grating, rather than stretching, to produce a measurement transducer for obtaining information on well and field integrity.
It would be enormously valuable in the art if there were a passive method of continually monitoring wells in real-time without physical intervention. It would constitute a great advance if it were possible to continually obtain precision, repeatable measurements. At any given time operators could be aware of well problems, particularly compaction, using highly reliable fiber gauges that should last the life of the well. It would be extremely valuable if there were also a method of passively monitoring in real-time to obtain data on compaction, 4D seismic, and information for geomechanical modeling.